Rear disc on A-body

For a streeter, with stock or near stock sized tires, I wouldn't bother.
The rear brakes do so little work, it's almost pathetic.
Go out and do three 30mph to zero stops, and measure the stopping distances.Average them out.
Then clamp the rear brake hose off and repeat. And remeasure. And calculate the percent difference.If the rear locks up, you may have a problem keeping the back in the back.And you already have too much brake in the back. But say that you are curious. Rip the P-valve out of your Combination valve, and bleed the air out.Repeat the 30 to zero test, now with full line pressure to the back. Lemme know what happens.